1. Field of the Invention
This invention relates to semiconductor switching devices and more particularly to heterojunction semiconductor devices which exhibit a negative resistance effect (i.e., wherein there is a decrease of current with increases in voltage).
2. Prior Art
One of the more well known negative resistance devices is the Gunn diode. That device is a bulk GaAs semiconductor which operates by a "transferred electron" mechanism. In most semiconductors the carriers reach a scattering limited velocity and the velocity vs. field plot saturates at a high field level. In some materials, however, e.g., GaAs, the energy of electrons can be raised by an applied field to the point that they transfer from one region of the conduction band to another higher energy region of the same conduction band. In n-type GaAs, the valence band is filled and the "central valley" or "minimum" of the conduction band normally contains the conduction electrons. There is a set of subsidiary minima or "satellite-valleys" at a higher energy within the conduction band but these are normally unoccupied by electrons.
If the GaAs is subjected to an electric field above some critical value, about 3,500 volts per cm, the electrons in the central valley gain more energy than the gap separating the central from the satellite valleys, (i.e., 360 meV) and there is considerable scattering into these higher energy satellite valleys. Once into the higher energy valleys, the electrons remain there as long as the field is greater than the critical value. Thus, once the field increases above the critical value, most conduction electrons in GaAs reside in the satellite valleys and exhibit properties typical of that region of the conduction band. In particular, the effective mass for electrons in the higher valleys is almost 20 times as great as in the central valley and the electron mobility is much lower. This is important when considering the negative conductivity mechanism since in the GaAs, as the electric field is increased, the electron velocity increases until a critical field is reached and then the electrons slow down with further increases in field. Obviously, while electrons are in the central valleys, they have a much higher effective mobility than when in the satellite valleys. Therefore, as more carriers transfer into the satellite valleys, and experience significant decreases in their mobility, the current passing through the GaAs decreases (even though there is increasing applied voltage) thus giving rise to the negative resistance characteristic. It is important to realize that this negative conductivity effect depends only on the bulk properties of the semiconductor and not on junction or surface effects.
On a related front, a line of semiconductor developments relating to semiconductor heterojunction structures has been progressing. Such structures generally involve either multilayer heterojunction arrangements or a periodic alternation of the doping of only one semiconductor to form a series of homojunctions. In U.S. Pat. No. 3,982,207, there is disclosed a heterostructure laser having a multilayer semiconductor body which includes an active region having a plurality of thin narrow band gap active layers interleaved with a plurality of thin relatively wider band gap passive layers. The passive layers are thin enough (indicated as being less than about 500 Angstroms) to permit carriers to distribute themselves among the active layers when the body is pumped either optically or electrically. The distribution of carriers is stated to occur either through tunneling or by hopping over the energy barriers created by the passive layers. The active layers are thin enough to separate the quantum levels of confined carriers. In this manner the patent indicates that the quantum size effects are exploited to produce wave length tunability without having to rely on changes of the composition of the active region and also to achieve lower lasing thresholds. It will be noted that the essence of that structure involves the application of pumping voltages in a manner orthogonal to the junction interfaces between the structures.
More recently, investigations of multilayer Al.sub.x Ga.sub.1-x As/GaAs/Al.sub.x Ga.sub.1-x As structures have shown that they can be constructed in such a manner as to provide a very high mobility in the GaAs layer which exceeds that of otherwise equivalent epitaxial GaAs. (Dingle, et. al., "Electron Mobilities in Modulation-Doped Semiconductor Heterojunction Super Lattices", Applied Physics Letter, 33 (7), Oct. 1, 1978, pp. 665-667).